Centrifuge rotors are fabricated from composite materials such as carbon fibers and carbon filaments, as well as with other fiber materials, bound into a suitable form with a polymerized synthetic resin. The selection of the fiber material, bound with the synthetic resin, is based on an exploitation of several unique physical properties of the fibers. For the application to centrifuge rotors, the most valuable physical property of any rotor construction material is a high strength to weight ratio. A second important property is high modulus of elasticity. A third desirable property is relatively low density. Composite structures, including centrifuge rotors, have been made with a number of different fiber materials such as glass filaments, boron filaments, synthetic organic fibers such as Dacron, and with various types of carbon fibers and carbon monofilaments. In general, the various forms of carbon fibers and filaments provide the most desirable combination of the above-mentioned properties for use in the construction of centrifuge rotors.
A rotor properly designed to exploit the above three properties will have an advantage over conventional metal rotors (usually aluminum or titanium alloys) in that, due to the high strength to weight ratio, there will be less centrifugally induced self stress and more of the material of the rotor is applied to supporting the centrifugal loads due to the samples which are being centrifuged. The high modulus of elasticity allows the composite (carbon fibers with the resin binder) to approach or match the modulus of metal parts which must be assembled to the rotor and to not deform excessively under centrifugal loads. Finally, the low density coupled with the high strength to weight ratio allows the design, for equivalent sample handling capacity, to be significantly lighter than metal rotors. This later advantage is particularly important for large volume rotors, which can be too heavy for a small lab technician to lift. At the same time, drive bearing life will markedly extended as rotor weight is reduced.
A further property of composite type materials and structures which has been exploited in centrifuge rotor and other composite structure design is the rather considerable possibility to orient the fibers in the composite so that they provide strength where it is needed. In metal construction, except for a moderate amount of anisotropic, properties which may be established by forging or selective heat treatment, the material has the same properties in all directions (isotropic) whether needed or not! Thus, as in U.S. Pat. No. 4,781,699 to A. Piramoon, a composite rotor is disclosed in which parts of the rotor are in the form of disks made up of multiple layers of monodirectional carbon fiber tape stacked and oriented so as to vary the angle of successive layers in increments of 45.degree. and bound together with a suitable resin. The object being to provide high strength at every direction in the plane of the disk but with strength normal to the plane only that due to the binding resin. The second composite material in this rotor construction consists of an outer ring of continuous fibers which are oriented primarily to provide hoop strength to give further support to the rotor is resisting the load due to the samples and sample holders.
Unfortunately, the design of composite material rotors is not without some special difficulties. One serious difficulty arises from the microphysical characteristics of the composite materials. Specifically, where it is necessary to provide a machined interface between composite and metal part, the surface characteristics of the composite requires quite special consideration. An examination of such a machined surface, even when finished by precision diamond grinding methods, reveals carbon fibers cut through at various angles, very large local variations in fiber versus binder areas and a considerable amount of imbedded ground fiber debris. It is quite apparent that this surface is not at all well suited to any sort of highly localized load bearing. It is, even with careful design measures taken to spread the load, very far from an ideal surface for the transmission of forces through frictional engagement or for withstanding highly oscillating and transient pulse loading. Because of these considerations, it has always been recognized that the conventional centrifuge drive hub socket, used with metal rotors, was not suitable for the composite materials. While it would be entirely feasible to machine the composite material, such as that of the lower disk of the above mentioned U.S. patent, to the same drive socket dimensions as used in an equivalent metal rotor; it is probable that the precision bore of the drive socket would be worn to an unacceptable degree after a very few cycles. The means for overcoming this problem has been to provide a metal drive hole so that the composite material machined interface is not subjected to the wear associated with the rotor drive hole. In the Piramoon patent the metal insert was designed to have an interference fit of approximately 0.0035" on diameter at room temperature and it was recognized that the interference would be as little as 0.0005" with the loaded rotor at 60,000 rpm. This interference fit was obtained by thermal shrinking of the insert in liquid nitrogen, installing it in the precision hole provided in the composite plate and allowing it to equilibrate to room temperature providing an expansion fit. In practice it has turned out that an interference fit of the needed dimensions in a composite material is difficult to make to the required reliability. This problem has been observed as a tendency for the insert to gradually shift its location by turning with respect to the rotor body and, eventually shifting so that the rotor becomes unbalanced. Clearly the interference fit between insert and rotor has been lost at running speed. Attempts to overcome this problem by providing pins against relative rotation have been partially successful. Even with the pins against rotation it is clear that there can be a certain amount of "working" on the diameter between the composite and the insert. It is well known that this sort of working at a frictional interface in a rotating system provides additional damping which, in turn, lowers the speed margin against the onset of high speed precession. Once this precession has started the working at the interface will progress rather rapidly and, in the instance of the rather fragile composite surface, can be expected to not only generate considerable local damage but, due to the changed surface and debris at the interface, rapidly increase the damping.